Assessing Methane Emissions From Tropical Wetlands: Uncertainties From Natural Variability and Drivers at the Global Scale

Methane (CH₄) emissions from tropical wetlands represent half of the global wetland emissions, but uncertainties remain concerning the extent of tropical methane sources. One limitation is to conceive tropical wetlands as a single ecosystem, especially in global land surface models. We estimate CH₄ emissions and assess their environmental and anthropogenic drivers. We created a data set with 101 studies involving 328-point measurements, classified the sites into four wetland types, and included relevant biological and environmental information. We estimate the global CH₄ emission rate from tropical wetlands as 35 (5–160) mg CH₄ m⁻² d⁻¹ (median, first and third quartile) and an annual global rate of 94 (56, 158) Tg y⁻¹. Fluxes differed among the wetland types, but except for anthropogenic factors, significant environmental drivers at the global scale could not be quantitatively identified because of high flux variability, even within wetland types. Coastal wetlands generate median emissions of 12 (5–23) Tg y⁻¹. Inland deep-water wetlands emit 53 (32–114) Tg y⁻¹, with highly variable areal extent. Emissions from inland shallow-water wetlands are 52 (33–78) Tg y⁻¹ with variation due to seasonal changes in water table level. Human-made wetlands emit 17 (10⁻⁴) Tg y⁻¹. Pollution and N inputs from agriculture are significant anthropogenic drivers of emissions from tropical wetlands. Specific drivers of change need to be considered according to wetland type when estimating global emissions as well as their specific vulnerability to global change. Additionally, these differences should be considered when implementing wetland management practices aimed at decreasing methane emissions.